1. Field of the Invention
The present invention relates to dry toner adapted for developing electrostatic or magnetic latent images in the electrophotographic, electrostatic printing, magnetic recording or other similar processes.
2. Description of the Prior Art
In the art of image formation such as electrophotography and electrostatic recording, there have been known various methods for visualizing or developing latent images electrically formed on the surface of a photosensitive member using a photoconductive material or on a dielectric or an insulating surface of an electrostatic recording member. For example, there may be mentioned the developing methods disclosed in U.S. Pat. Nos. 2,576,047 and 3,081,698 and British Pat. Nos. 1,165,406 and 1,165,405.
In these known developing methods, the development of electorstatic latent image is generally carried out by using electroscopic particles which are selectively attracted or repulsed by the electrostatic charges of the latent image although there is some difference in technique depending upon whether direct reproduction is desired or reversal reproduction is desired. Such electroscopic particles used in developing are usually called "toner". In case of direct reproduction, the tone adheres to the area in which the latent image is present. On the contrary, in case of reversal reproduction, it adheres to the area in which the latent is not present.
The toner can be applied to a latent image carrier surface in various manners to effect developing the latent image. In dry developing process which is most popular at present and in which toner in a form of dry powder is used, there are known and used a magnetic brush method as disclosed in U.S. Pat. No. 2,874,063, a cascade method as disclosed in U.S. Pat. No. 2,618,552 and a powder clouding method as disclosed in U.S. Pat. No. 2,221,776.
Developing agents used in these developing methods may be classified into two groups, that is, those of unitary system comprising solely colored particles, namely toner and those of binary system comprising toner and carrier such as iron powder or glass beads. The first mentioned developing agent is able to develop the latent image by the aid of electric charge derived from an electrically conductive member such as a magnetic metal sleeve used for carrying the developer. The latter mentioned type of the developer is able to develop the latent image by the aid of electric charge derived from frictional charging. A well-known developing method using the unitary system developer is that in which an electrically conductive toner is used. This is disclosed, for example, in U.S. Pat. No. 3,909,258 and Japanese Patent Publication No. 491/1962. According to the method, toner having electric conductivity and magnetism is adhered to a sleeve having a magnet mounted therein so as to form a magnetic brush of toner. The magnetic brush of toner thus formed is brought into contact with an electrostatic latent image carrier to effect developing the latent image with the toner. Since the toner is electrically conductive, when the magnetic brush is placed opposed to the electrostatic latent image, electric charge of opposite polarity to that of the electrostatic latent image is induced in the toner so that the latent image can be developed under the action of electric attraction force existing between the toner having the induced charge and the electrostatic latent image.
The above described developing method using the unitary system of developing agent containing only toner has various advantages as compared to the methods using the binary system of developing agent. When the binary system developing agent is used, it is required to adjust the mixing ratio of toner and carrier, that is, the concentration of toner in the developer. In case of the developing method using the unitary system of developing agent, there is no need of such adjustment. Moreover, it needs no mixing operation of developer which is otherwise required to electrically charge the developer. Therefore, the developing method can be carried out with a developing apparatus which is simple in structure and compact in arrangement. Another advantage of the above described developing method is found in the fact that it has no problem of the quality degradation of developed images caused by the deterioration of carrier with the lapse of time.
While the above described developing method has such various advantages over other developing methods, its use has been limited only to the case wherein an electrostatic latent image is formed on a photosensitive paper coated with a photosensitive material such as zinc oxide and the latent image is developed directly with toner and the developed image is fixed on the paper. However, the developing system which is used most widely in the art at present comprises a transferring step in which a developed image is transferred to a trasnfer sheet such as an ordinary paper from the photosensitive medium. The above described method can not be applied to this developing system without a great difficulty and inconvenience. This is because electrically conductive toner which gives a limitation to the transferring operation is used in the above described developing method. For example, when such conductive toner is used in the corona transferring process, it gives rise to many difficulties. The corona transferring process is a transferring process in which a toner image formed on an electorstatic latent image carrier is brought into contact with a transfer sheet and an electric charge is applied from the backside surface of the transfer sheet by corona discharge to effect transferring the toner image. In this process, if the toner is electrically conductive, then a weak corona current passing through the transfer sheet will make the toner particles have the same polarity of electric charge as the transfer sheet has. This results in an insufficient transfer of the toner image. Furthermore, since all the toner particles in a toner layer are made to have electric charges of the same polarity, developed images may be disturbed by the repulsion among the toner particles. Japanese Patent Application Laid Open No. 45639/1975 describes a developing method of elestrostatic latent images in which an electrically insulating and magnetic toner is used and the toner is charged by using the friction electricity charging effect produced between the toner and the surface of toner supporting member to develop the electrostatic latent image. According to the description of the specification of this patent application, it is also possible to electrostatically transfer the developed toner image. However, because of the high content of magnetic material dispsersed therein in a form of fine particles, this insulating magnetic toner is not so good or not so stable with respect to its chargeability. The triboelectric charge thereof often varies during the use, and therefore, it is difficult to repeatedly produce good images in a stable manner.
It is also known to add an electric charge controlling agent such as Nigrosine dye or metal containing dye to a magnetic toner, but the addition of such electric charge controlling agent to the magnetic toner brings forth some problems. In making a large number of copies, dye contained in the magnetic toner becomes adhered to or absorbed to the surface of toner supporting member more and more and finally the friction chargeability of toner is reduced extremely. For this reason, use of the charge controlling dye is not preferable.